1. Technical Field
The present invention generally relates to threshold voltage control for semiconductor devices, and, in particular, to common fabrication of mixed-type non-planar semiconductor devices having different threshold voltages.
2. Background Information
Current schemes for controlling threshold (i.e., turn-on) voltage (Vt) in semiconductor devices, such as transistors, include doping the channel. Although convenient, Vt control by channel doping quickly begins to degrade performance beyond very low levels of doping. Another current scheme to control Vt is the use of one or more layers of work function metal to achieve a desired Vt. However, this method is generally unstable for p-type devices, and the use of work function metal with n-type devices may further affect neighboring p-type devices. Thus, where mixed-type devices (i.e., n-type and p-type) are co-fabricated, using layers of work function metal alone is problematic. In addition, as device size continues to shrink, the amount of space in a gate stack remaining for the work function metal also decreases. Thus, the use of work function metal alone to adjust Vt has decreasing utility as device size decreases.
Therefore, a need continues to exist for threshold voltage control for mixed-type non-planar semiconductor devices.